Directional asymmetry in the volume of the human habenula

Habenula contributions to negative self-cognitions

Self-related cognitions are integral to personal identity and psychological wellbeing. Persistent engagement with negative self-cognitions can precipitate mental ill health; whereas the ability to restructure them is protective. Here, we leverage ultra-high field 7T fMRI and dynamic causal modelling to characterise a negative self-cognition network centred on the habenula - a small midbrain region linked to the encoding of punishment and negative outcomes. We model habenula effective connectivity in a discovery sample of healthy young adults (n = 45) and in a replication cohort (n = 56) using a cognitive restructuring task during which participants repeated or restructured negative self-cognitions. The restructuring of negative self-cognitions elicits an excitatory effect from the habenula to the posterior orbitofrontal cortex that is reliably observed across both samples. Furthermore, we identify an excitatory effect of the habenula on the posterior cingulate cortex during both the repeating and restructuring of self-cognitions. Our study provides evidence demonstrating the habenula's contribution to processing self-cognitions. These findings yield unique insights into habenula's function beyond processing external reward/punishment to include abstract internal experiences.

The lamprey habenula provides an extreme example for the temporal regulation of asymmetric development

By their phylogenetic position and their marked epithalamic asymmetries, lampreys are relevant models for understanding the formation and evolution of this trait across vertebrates. In this study, we use a transcriptomic approach to identify novel signature markers to characterize the highly asymmetric, bipartite organization of habenulae in lampreys. Lamprey habenulae are subdivided into two complementary subdomains related, respectively, to the lateral/ventral and the medial/dorsal habenulae of jawed vertebrates: a dorsal, right-restricted subdomain and a bilateral subdomain that includes the left habenula as well as its ventral right counterpart. Analysis of the formation of the lamprey habenula at prolarval and larval stages using a combination of morphological, immunohistochemical, and in situ hybridization approaches highlights a marked asymmetric temporal regulation. The dorsal right subdomain forms and already expresses all identified signature markers in prolarval stages. In contrast, the left and ventral right subdomain appears significantly later, with the first indication of neuronal identity elaboration in these territories being observed in larval stages. As in gnathostomes, Wnt signaling may be involved in the regulation of this unique, asymmetric mode of development, since β-catenin shows asymmetric and highly dynamic nuclear distributions both in neural progenitors and differentiated neuronal precursors of the two habenular subdomains. These data confirm the importance of lampreys to unravel the developmental logic underlying the recurrence and variation of habenular asymmetries in vertebrates and pave the way for future functional analyses.

The role of sleep quality in mediating the relationship between habenula volume and resilience

BACKGROUND

Our human volumetric MRI study (Dai et al., 2024) demonstrated that habenula (Hb) volume is associated with psychological resilience, a key protective factor against depression. However, the biological mechanisms underpinning this relationship remain unclear. A recent animal study highlighted that neuronal activity in the Hb modulates rapid eye movement (REM) sleep, influencing depressive behaviors during wakefulness. Based on this, we hypothesized that sleep quality mediates the relationship between Hb volume and psychological resilience in humans.

METHODS

We utilized a deep learning-based automated segmentation model to estimate Hb volume from 3T-MRI T1-weighted images of 84 healthy participants. Correlation analyses were performed to examine the relationship between Hb volume and questionnaire-based assessments of sleep quality. Mediation analysis was then conducted with Hb volume as the independent variable, psychological resilience as the dependent variable, and sleep quality as the mediator.

RESULTS

Hb volume was found to be negatively correlated with sleep disturbance, indicating that individuals with larger Hb volumes experienced better sleep quality. A lateralization effect was also observed, where greater leftward asymmetry (larger left Hb volume compared to right) was associated with more severe sleep disturbances. Moreover, sleep quality was identified as a mediator in the relationship between Hb volume and psychological resilience.

CONCLUSION

This study provides preliminary evidence of the association among Hb volume, sleep quality, and resilience. Sleep quality appears to be a critical mediator in the biological processes linking smaller Hb volumes to decreased psychological resilience. Enhancing sleep quality may be a promising approach for bolstering psychological resilience and reducing the risk of depression.

Analysis of a shark reveals ancient, Wnt-dependent, habenular asymmetries in vertebrates

The mode of evolution of left-right asymmetries in the vertebrate habenulae remains largely unknown. Using a transcriptomic approach, we show that in a cartilaginous fish, the catshark Scyliorhinus canicula, habenulae exhibit marked asymmetries, in both their medial and lateral components. Comparisons across vertebrates suggest that those identified in lateral habenulae reflect an ancestral gnathostome trait, partially conserved in lampreys, and independently lost in tetrapods and neopterygians. Asymmetry formation involves distinct mechanisms in the catshark lateral and medial habenulae. Medial habenulae are submitted to a marked, asymmetric temporal regulation of neurogenesis, undetectable in their lateral counterparts. Conversely, asymmetry formation in lateral habenulae results from asymmetric choices of neuronal identity in post-mitotic progenitors, a regulation dependent on the repression of Wnt signaling by Nodal on the left. Based on comparisons with the mouse and the zebrafish, we propose that habenular asymmetry formation involves a recurrent developmental logic across vertebrates, which relies on conserved, temporally regulated genetic programs sequentially shaping choices of neuronal identity on both sides and asymmetrically modified by Wnt activity.

Psychological resilience is positively correlated with Habenula volume

BACKGROUND

Psychological resilience is defined as the process and outcome of individuals' successful adaptation to challenging life experiences. The Habenula (Hb) is known to be involved in the stress response; however, the relationship between Hb volume and resilience in humans remains unclear. This study investigated the correlation among resilience, Hb volume, and depressive tendencies in adults.

METHODS

Hb volumes were assessed using deep learning techniques applied to 110 healthy participants. Resilience and depression were evaluated using the Connor-Davidson Resilience Scale and Beck Depression Inventory-II, respectively. We examined the relationship between Hb volume and resilience and assessed the mediating effects of resilience on the relationship between Hb volume and depressive tendencies.

RESULTS

Correlation analysis revealed a positive correlation between resilience and Hb volume (partial r = 0.176, p = 0.001), which was more pronounced in women (partial r = 0.353, p = 0.003). Hb volumes on the left and right sides exhibited significant lateralization (LI = 0.031, 95 % CI = [0.016, 0.046]). Despite Hb asymmetry, lateralization was not significantly associated with resilience. The mediation analysis shows significant indirect effect of resilience on the relationship between Hb volume and depressive tendencies (β = -0.093, 95%CI = [-0.189, -0.019]).

CONCLUSION

This study found that populations with lower resilience have smaller Hb volume. Previous research has shown that Hb volume decreased with the increasing severity of depression symptoms in patients. Our findings support this view and extend it to a population that has not been clinically diagnosed with depression. Additionally, we found that psychological resilience can be predicted by Hb volume and may serve as a mediating factor indirectly affecting depressive tendencies, even in healthy individuals.

LIMITATIONS

Due to its cross-sectional design, this study was unable to analyze dynamic changes in Hb volume during the process of resilience adaptation.

Habenula volume change in Parkinson's disease: A 7T MRI study

OBJECTIVE

Parkinson's disease (PD) is a progressive neurodegenerative disease characterized by motor and early non-motor symptoms. The habenula is implicated in the pathophysiology of depression. This study investigates habenular volume in PD patients without clinical depression to show the changes in PD unrelated to depression.

METHODS

The study used high-resolution 7 Tesla MRI data from the TRACK-PD study involving 104 PD patients and 44 healthy controls (HCs). The habenula was manually segmented, and volumes were measured, considering demographic data and depression scores via the Beck Depression Inventory (BDI).

RESULTS

No significant correlation was found between habenular volume and BDI scores in PD patients or HCs. However, the PD group exhibited a significantly larger mean and right habenular volume than HCs. Although PD patients showed higher BDI scores, indicating more subthreshold depression, these did not correlate with the habenular volume.

CONCLUSION

The results suggest that while the habenula may be involved in the symptoms of PD, its role in depression within this cohort is unclear. The changes might be related to the role of the habenula in motor symptoms. This study provides a new perspective on the role of the habenula in PD, but future research could lead to a greater understanding of the neuroanatomical features of the habenula in PD.

The behavioral relevance of a modular organization in the lateral habenula

Behavioral strategies for survival rely on the updates the brain continuously makes based on the surrounding environment. External stimuli-neutral, positive, and negative-relay core information to the brain, where a complex anatomical network rapidly organizes actions, including approach or escape, and regulates emotions. Human neuroimaging and physiology in nonhuman primates, rodents, and teleosts suggest a pivotal role of the lateral habenula in translating external information into survival behaviors. Here, we review the literature describing how discrete habenular modules-reflecting the molecular signatures, anatomical connectivity, and functional components-are recruited by environmental stimuli and cooperate to prompt specific behavioral outcomes. We argue that integration of these findings in the context of valence processing for reinforcing or discouraging behaviors is necessary, offering a compelling model to guide future work.

Segmentation and Volume Estimation of the Habenula Using Deep Learning in Patients With Depression

Background

The habenula is involved in the pathophysiology of depression. However, its small structure limits the accuracy of segmentation methods, and the findings regarding its volume have been inconsistent. This study aimed to create a highly accurate habenula segmentation model using deep learning, test its generalizability to clinical magnetic resonance imaging, and examine differences between healthy participants and patients with depression.

Methods

This multicenter study included 382 participants (patients with depression: N = 234, women 47.0%; healthy participants: N = 148, women 37.8%). A 3-dimensional residual U-Net was used to create a habenula segmentation model on 3T magnetic resonance images. The reproducibility and generalizability of the predictive model were tested on various validation cohorts. Thereafter, differences between the habenula volume of healthy participants and that of patients with depression were examined.

Results

A Dice coefficient of 86.6% was achieved in the derivation cohort. The test-retest dataset showed a mean absolute percentage error of 6.66, indicating sufficiently high reproducibility. A Dice coefficient of >80% was achieved for datasets with different imaging conditions, such as magnetic field strengths, spatial resolutions, and imaging sequences, by adjusting the threshold. A significant negative correlation with age was observed in the general population, and this correlation was more pronounced in patients with depression (p < 10-7, r = -0.59). Habenula volume decreased with depression severity in women even when the effects of age and scanner were excluded (p = .019, η2 = 0.099).

Conclusions

Habenula volume could be a pathophysiologically relevant factor and diagnostic and therapeutic marker for depression, particularly in women.

Recent Advances in Habenula Imaging Technology: A Comprehensive Review

The habenula (Hb) is involved in many natural human behaviors, and the relevance of its alterations in size and neural activity to several psychiatric disorders and addictive behaviors has been presumed and investigated in recent years using magnetic resonance imaging (MRI). Although the Hb is small, an increasing number of studies have overcome the difficulties in MRI. Conventional structural-based imaging also has great defects in observing the Hb contrast with adjacent structures. In addition, more and more attention should be paid to the Hb's functional, structural, and quantitative imaging studies. Several advanced MRI methods have recently been employed in clinical studies to explore the Hb and its involvement in psychiatric diseases. This review summarizes the anatomy and function of the human Hb; moreover, it focuses on exploring the human Hb with noninvasive MRI approaches, highlighting strategies to overcome the poor contrast with adjacent structures and the need for multiparametric MRI to develop imaging markers for diagnosis and treatment follow-up. LEVEL OF EVIDENCE: 3 TECHNICAL EFFICACY STAGE: 2.

The habenula in Parkinson's disease: Anatomy, function, and implications for mood disorders - A narrative review

Parkinson's disease (PD), a widespread neurodegenerative disorder, often coexists with mood disorders. Degeneration of serotonergic neurons in brainstem raphe nuclei have been linked to depression and anxiety. Additionally, the locus coeruleus and its noradrenergic neurons are among the first areas to degenerate in PD and contribute to stress, emotional memory, motor, sensory, and autonomic symptoms. Another brain region of interest is habenula, which is especially related to anti-reward processing, and its function has recently been linked to PD and to mood-related symptoms. There are several neuroimaging studies that investigated role of the habenula in mood disorders. Differences in habenular size and hemispheric symmetry were found in healthy controls compared to individuals with mood disorders. The lateral habenula, as a link between the dopaminergic and serotonergic systems, is thought to contribute to depressive symptoms in PD. However, there is only one imaging study about role of habenula in mood disorders in PD, although the relationship between PD and mood disorders is known. There is little known about habenula pathology in PD but given these observations, the question arises whether habenular dysfunction could play a role in PD and the development of PD-related mood disorders. In this review, we evaluate neuroimaging techniques and studies that investigated the habenula in the context of PD and mood disorders. Future studies are important to understand habenula's role in PD patients with mood disorders. Thus, new potential diagnostic and treatment opportunities would be found for mood disorders in PD.

The Neuroanatomy of the Habenular Complex and Its Role in the Regulation of Affective Behaviors

The habenular complex is a diencephalic structure divided into the medial and lateral divisions that lie within the epithalamus of most vertebrates. This brain structure, whose activities are mainly regulated via inputs/outputs from and to the stria medullaris and the fasciculus retroflexus, plays a significant role in the modulation of anti-reward behaviors in both the rodent and human brain. Such anti-reward circuits are regulated by dopaminergic and serotonergic projections with several other subcortical and cortical regions; therefore, it is plausible that impairment to this key subcortical structure or its connections contributes to the pathogenesis of affective disorders. Current literature reveals the existence of structural changes in the habenula complex in individuals afflicted by such disorders; however, there is a need for more comprehensive investigations to elucidate the underlying neuroanatomical connections that underpin disease development. In this review article, we aim to provide a comprehensive view of the neuroanatomical differences between the rodent and human habenular complex, the main circuitries, and provide an update on the emerging roles of this understudied subcortical structure in the control of affective behaviors, with special emphasis to morbid conditions of the affective sphere.

Understanding the habenula: A major node in circuits regulating emotion and motivation

Over the last decade, the understanding of the habenula has rapidly advanced from being an understudied brain area with the Latin name 'habena" meaning "little rein", to being considered a "major rein" in the control of key monoaminergic brain centers. This ancient brain structure is a strategic node in the information flow from fronto-limbic brain areas to brainstem nuclei. As such, it plays a crucial role in regulating emotional, motivational, and cognitive behaviors and has been implicated in several neuropsychiatric disorders, including depression and addiction. This review will summarize recent findings on the medial (MHb) and lateral (LHb) habenula, their topographical projections, cell types, and functions. Additionally, we will discuss contemporary efforts that have uncovered novel molecular pathways and synaptic mechanisms with a focus on MHb-Interpeduncular nucleus (IPN) synapses. Finally, we will explore the potential interplay between the habenula's cholinergic and non-cholinergic components in coordinating related emotional and motivational behaviors, raising the possibility that these two pathways work together to provide balanced roles in reward prediction and aversion, rather than functioning independently.

The habenular volume and PDE7A allelic polymorphism in major depressive disorder: preliminary findings

OBJECTIVES

The habenula is a brain structure implicated in depression, yet with unknown molecular mechanisms. Several phosphodiesterases (PDEs) have been associated with a risk of depression. Although the role of PDE7A in the brain is unknown, it has enriched expression in the medial habenula, suggesting that it may play a role in depression.

METHODS

We analysed: (1) habenula volume assessed by 3-T magnetic resonance imaging (MRI) in 84 patients with major depressive disorder (MDD) and 41 healthy controls; (2) frequencies of 10 single nucleotide polymorphisms (SNPs) in PDE7A gene in 235 patients and 41 controls; and (3) both indices in 80 patients and 27 controls. The analyses considered gender, age, body mass index and season of the MRI examination.

RESULTS

The analysis did not reveal habenula volumetric changes in MDD patients regardless of PDE7A SNPs. However, in the combined group, the carriers of one or more mutations among 10 SNPs in the PDE7A gene had a lower volume of the left habenula (driven mainly by rs972362 and rs138599850 mutations) and consequently had the reduced habenular laterality index in comparison with individuals without PDE7A mutations.

CONCLUSIONS

Our findings suggest the implication of the PDE7A gene into mechanisms determining the habenula structure.

Circuits regulating pleasure and happiness - focus on potential biomarkers for circuitry including the habenuloid complex

INTRODUCTION

The multiplicity and complexity of the neuronal connections in the central nervous system make it difficult to disentangle circuits that play an essential role in the development or treatment of (neuro)psychiatric disorders. By choosing the evolutionary development of the forebrain as a starting point, a certain order in the connections can be created. The dorsal diencephalic connection (DDC) system can be applied for the development of biomarkers that can predict treatment response.

MATERIALS AND METHODS

After providing a brief introduction to the theory, we examined neuroanatomical publications on the connectivity of the DDC system. We then searched for neurochemical components that are specific for the habenula.

RESULTS AND DISCUSSION

The best strategy to find biomarkers that reflect the function of the habenular connection is to use genetic variants of receptors, transporters or enzymes specific to this complex. By activating these with probes and measuring the response in people with different functional genotypes, the usefulness of biomarkers can be assessed.

CONCLUSIONS

The most promising biomarkers in this respect are those linked to activation or inhibition of the nicotine receptor, dopamine D4 receptor, μ-opioid receptor and also those of the functioning of habenular glia cells (astrocytes and microglia).

Laterality and Sex Differences of Human Lateral Habenula Afferent and Efferent Fiber Tracts

Introduction

The lateral habenula (LHb) is an epithalamic nucleus associated with negative valence and affective disorders. It receives input via the stria medullaris (SM) and sends output via the fasciculus retroflexus (FR). Here, we use tractography to reconstruct and characterize this pathway.

Methods

Multi-shell human diffusion magnetic resonance imaging (dMRI) data was obtained from the human connectome project (HCP) (n = 20, 10 males) and from healthy controls (n = 10, 6 males) scanned at our institution. We generated LHb afferents and efferents using probabilistic tractography by selecting the pallidum as the seed region and the ventral tegmental area as the output target.

Results

We were able to reconstruct the intended streamlines in all individuals from the HCP dataset and our dataset. Our technique also aided in identification of the LHb. In right-handed individuals, the streamlines were significantly more numerous in the left hemisphere (mean ratio 1.59 ± 0.09, p = 0.04). In left-handed individuals, there was no hemispheric asymmetry on average (mean ratio 1.00 ± 0.09, p = 1.0). Additionally, these streamlines were significantly more numerous in females than in males (619.9 ± 159.7 vs. 225.9 ± 66.03, p = 0.04).

Conclusion

We developed a method to reconstruct the SM and FR without manual identification of the LHb. This technique enables targeting of these fiber tracts as well as the LHb. Furthermore, we have demonstrated that there are sex and hemispheric differences in streamline number. These findings may have therapeutic implications and warrant further investigation.

Cerebral Polymorphisms for Lateralisation: Modelling the Genetic and Phenotypic Architectures of Multiple Functional Modules

Recent fMRI and fTCD studies have found that functional modules for aspects of language, praxis, and visuo-spatial functioning, while typically left, left and right hemispheric respectively, frequently show atypical lateralisation. Studies with increasing numbers of modules and participants are finding increasing numbers of module combinations, which here are termed cerebral polymorphisms—qualitatively different lateral organisations of cognitive functions. Polymorphisms are more frequent in left-handers than right-handers, but it is far from the case that right-handers all show the lateral organisation of modules described in introductory textbooks. In computational terms, this paper extends the original, monogenic McManus DC (dextral-chance) model of handedness and language dominance to multiple functional modules, and to a polygenic DC model compatible with the molecular genetics of handedness, and with the biology of visceral asymmetries found in primary ciliary dyskinesia. Distributions of cerebral polymorphisms are calculated for families and twins, and consequences and implications of cerebral polymorphisms are explored for explaining aphasia due to cerebral damage, as well as possible talents and deficits arising from atypical inter- and intra-hemispheric modular connections. The model is set in the broader context of the testing of psychological theories, of issues of laterality measurement, of mutation-selection balance, and the evolution of brain and visceral asymmetries.

Multi-level variations of lateral habenula in depression: A comprehensive review of current evidence

Despite extensive research in recent decades, knowledge of the pathophysiology of depression in neural circuits remains limited. Recently, the lateral habenula (LHb) has been extensively reported to undergo a series of adaptive changes at multiple levels during the depression state. As a crucial relay in brain networks associated with emotion regulation, LHb receives excitatory or inhibitory projections from upstream brain regions related to stress and cognition and interacts with brain regions involved in emotion regulation. A series of pathological alterations induced by aberrant inputs cause abnormal function of the LHb, resulting in dysregulation of mood and motivation, which present with depressive-like phenotypes in rodents. Herein, we systematically combed advances from rodents, summarized changes in the LHb and related neural circuits in depression, and attempted to analyze the intrinsic logical relationship among these pathological alterations. We expect that this summary will greatly enhance our understanding of the pathological processes of depression. This is advantageous for fostering the understanding and screening of potential antidepressant targets against LHb.

Decreased resting-state functional connectivity of the habenula-cerebellar in a major depressive disorder

BACKGROUND

In animal experiments, the habenula and septal nuclei are known as the key brain areas of depression. However, there are few magnetic resonance imaging (MRI) studies on the functional connectivity between these areas and the subcortical areas in humans with major depression. We aimed to investigate the difference in resting-state functional connectivity (RSFC) among the major regions of interest (ROI) in the subcortical areas, including both the habenula and septal nuclei.

METHODS

We performed the seed-to-voxel analysis to investigate the RSFC between both the habenula and septal nucleus, as well as other subcortical regions. Furthermore, ROI-to-ROI analysis was performed among the combinations of ROI pairs in the subcortical areas.

RESULTS

The seed-to-voxel analysis showed a lower RSFC between the left habenula and the cerebellum in major depressive disorder (MDD) than in healthy controls (HCs). As a result of ROI-to-ROI analysis in subcortical areas, a total of 31 pairs of FCs in the MDD group showed a lower RSFC than in the HCs group.

CONCLUSION

This study revealed a lower RSFC between the left habenula and cerebellum in patients with MDD and reduced RSFC among numerous subcortical areas. These new findings on the neural circuitry of MDD might contribute to an in-depth understanding of depression.

Left-right asymmetric and smaller right habenula volume in major depressive disorder on high-resolution 7-T magnetic resonance imaging

The habenula (Hb) has been hypothesized to play an essential role in major depressive disorder (MDD) as it is considered to be an important node between fronto-limbic areas and midbrain monoaminergic structures based on animal studies. In this study, we aimed to investigate the differences in volume and T1 value of the Hb between patients with MDD and healthy control (HC) subjects. Analysis for the Hb volumes was performed using high-resolution 7-T magnetic resonance (MR) image data from 33 MDD patients and 36 healthy subjects. Two researchers blinded to the clinical data manually delineated the habenular nuclei and Hb volume, and T1 values were calculated based on overlapping voxels. We compared the Hb volume and T1 value between the MDD and HC groups and compared the volume and T1 values between the left and right Hbs in each group. Compared to HC subjects, MDD patients had a smaller right Hb volume; however, there was no significant volume difference in the left Hb between groups. In the MDD group, the right Hb was smaller in volume and lower in T1 value than the left Hb. The present findings suggest a smaller right Hb volume and left-right asymmetry of Hb volume in MDD. Future high-resolution 7-T MR imaging studies with larger sample sizes will be needed to derive a more definitive conclusion.

Reproducibility of automated habenula segmentation via deep learning in major depressive disorder and normal controls with 7 Tesla MRI

The habenula is one of the most important brain regions for investigating the etiology of psychiatric diseases such as major depressive disorder (MDD). However, the habenula is challenging to delineate with the naked human eye in brain imaging due to its low contrast and tiny size, and the manual segmentation results vary greatly depending on the observer. Therefore, there is a great need for automatic quantitative analytic methods of the habenula for psychiatric research purposes. Here we propose an automated segmentation and volume estimation method for the habenula in 7 Tesla magnetic resonance imaging based on a deep learning-based semantic segmentation network. The proposed method, using the data of 69 participants (33 patients with MDD and 36 normal controls), achieved an average precision, recall, and dice similarity coefficient of 0.869, 0.865, and 0.852, respectively, in the automated segmentation task. Moreover, the intra-class correlation coefficient reached 0.870 in the volume estimation task. This study demonstrates that this deep learning-based method can provide accurate and quantitative analytic results of the habenula. By providing rapid and quantitative information on the habenula, we expect our proposed method will aid future psychiatric disease studies.

Diffusion Tensor Imaging: A Promising New Technique for Accurate Identification of the Stria Medullaris and Habenula

The Stria Medullaris (SM) is a white-matter tract that contains afferent fibres that connect the cognitive-emotional areas in the forebrain to the Habenula (Hb). The Hb plays an important role in behavioral responses to reward, stress, anxiety, pain, and sleep through its action on neuromodulator systems. The Fasciculus Retroflexus (FR) forms the primary output of the Hb to the midbrain. The SM, Hb, and FR are part of a special pathway between the forebrain and the midbrain known as the Dorsal Diencephalic Conduction system (DDC). Hb dysfunction is accompanied by different types of neuropsychiatric disorders, such as schizophrenia, depression, and Treatment-Resistant Depression (TRD). Due to difficulties in the imaging assessment of the SM and HB in vivo, they had not been a focus of clinical studies until the invention of Diffusion Tensor Imaging (DTI), which has revolutionized the imaging and investigation of the SM and Hb. DTI has facilitated the imaging of the SM and Hb and has provided insights into their properties through the investigation of their monoamine dysregulation. DTI is a well-established technique for mapping brain microstructure and white matter tracts; it provides indirect information about the microstructural architecture and integrity of white matter in vivo, based on water diffusion properties in the intra- and extracellular space, such as Axial Diffusivity (AD), Radial Diffusivity (RD), mean diffusivity, and Fractional Anisotropy (FA). Neurosurgeons have recognized the potential value of DTI in the direct anatomical targeting of the SM and Hb prior to Deep Brain Stimulation (DBS) surgery for the treatment of certain neuropsychiatric conditions, such as TRD. DTI is the only non-invasive method that offers the possibility of visualization in vivo of the white-matter tracts and nuclei in the human brain. This review study summarizes the use of DTI as a promising new imaging method for accurate identification of the SM and Hb, with special emphasis on direct anatomical targeting of the SM and Hb prior to DBS surgery.

Zebrafish Tools for Deciphering Habenular Network-Linked Mental Disorders

Simple Summary

Everything that we think, feel or do depends on the function of neural networks in the brain. These are highly complex structures made of cells (neurons) and their interconnections (axons), which develop dependent on precisely coordinated interactions of genes. Any gene mutation can result in unwanted alterations in neural network formation and concomitant brain disorders. The habenula neural network is one of these important circuits, which has been linked to autism, schizophrenia, depression and bipolar disorder. Studies using the zebrafish have uncovered genes involved in the development of this network. Intriguingly, some of these genes have also been identified as risk genes of human brain disorders highlighting the power of this animal model to link risk genes and the affected network to human disease. But can we use the advantages of this model to identify new targets and compounds with ameliorating effects on brain dysfunction? In this review, we summarise the current knowledge on techniques to manipulate the habenula neural network to study the consequences on behavior. Moreover, we give an overview of existing behavioral test to mimic aspects of mental disorders and critically discuss the applicability of the zebrafish model in this field of research.

Abstract

The prevalence of patients suffering from mental disorders is substantially increasing in recent years and represents a major burden to society. The underlying causes and neuronal circuits affected are complex and difficult to unravel. Frequent disorders such as depression, schizophrenia, autism, and bipolar disorder share links to the habenular neural circuit. This conserved neurotransmitter system relays cognitive information between different brain areas steering behaviors ranging from fear and anxiety to reward, sleep, and social behaviors. Advances in the field using the zebrafish model organism have uncovered major genetic mechanisms underlying the formation of the habenular neural circuit. Some of the identified genes involved in regulating Wnt/beta-catenin signaling have previously been suggested as risk genes of human mental disorders. Hence, these studies on habenular genetics contribute to a better understanding of brain diseases. We are here summarizing how the gained knowledge on the mechanisms underlying habenular neural circuit development can be used to introduce defined manipulations into the system to study the functional behavioral consequences. We further give an overview of existing behavior assays to address phenotypes related to mental disorders and critically discuss the power but also the limits of the zebrafish model for identifying suitable targets to develop therapies.

Thalamo-Habenular Connection Differences Between Patients With Major Depressive Disorder and Normal Controls

Background: The thalamus and habenula are thought to be key brain regions in the etiology of major depressive disorder (MDD); however, few studies have investigated the structural connection between them. We compared the number of white matter tracts between the thalamus and habenula between patient with MDD and normal controls (NCs). Methods: The habenula and thalamus region of interest masks were extracted from brain magnetic resonance imaging data and individual tractography analysis was performed. First, we compared the number of fiber connections from the habenula to the thalamus between the MDD (n = 34) and NC (n = 37) groups and also compared hemispherical differences to investigate possible asymmetries. Results: There was a significant difference in the number of tracts in the right habenula-left mediodorsal thalamus pair between the two groups. For hemispherical fiber connections, the waytotal ratio of the right ipsilateral tract between the thalamus and habenula was significantly higher than that of the left ipsilateral tract in both groups. Conclusion: The number of right habenula-left mediodorsal thalamus tracts was higher in patients with MDD than in NCs. These results indicate that MDD is related to the disintegration of the left thalamus-right habenula tract function with an increased number of tracts as a compensational mechanism.

Importance of the Habenula for Avoidance Learning Including Contextual Cues in the Human Brain: A Preliminary fMRI Study

Human habenula studies are gradually advancing, primarily through the use of functional magnetic resonance imaging (fMRI) analysis of passive (Pavlovian) conditioning tasks as well as probabilistic reinforcement learning tasks. However, no studies have particularly targeted aversive prediction errors, despite the essential importance for the habenula in the field. Complicated learned strategies including contextual contents are involved in making aversive prediction errors during the learning process. Therefore, we examined habenula activation during a contextual learning task. We performed fMRI on a group of 19 healthy controls. We assessed the manually traced habenula during negative outcomes during the contextual learning task. The Beck Depression Inventory-Second Edition (BDI-II), the State-Trait-Anxiety Inventory (STAI), and the Temperament and Character Inventory (TCI) were also administered. The left and right habenula were activated during aversive outcomes and the activation was associated with aversive prediction errors. There was also a positive correlation between TCI reward dependence scores and habenula activation. Furthermore, dynamic causal modeling (DCM) analyses demonstrated the left and right habenula to the left and right hippocampus connections during the presentation of contextual stimuli. These findings serve to highlight the neural mechanisms that may be relevant to understanding the broader relationship between the habenula and learning processes.

Untangling the dorsal diencephalic conduction system: a review of structure and function of the stria medullaris, habenula and fasciculus retroflexus

The often-overlooked dorsal diencephalic conduction system (DDCS) is a highly conserved pathway linking the basal forebrain and the monoaminergic brainstem. It consists of three key structures; the stria medullaris, the habenula and the fasciculus retroflexus. The first component of the DDCS, the stria medullaris, is a discrete bilateral tract composed of fibers from the basal forebrain that terminate in the triangular eminence of the stalk of the pineal gland, known as the habenula. The habenula acts as a relay hub where incoming signals from the stria medullaris are processed and subsequently relayed to the midbrain and hindbrain monoaminergic nuclei through the fasciculus retroflexus. As a result of its wide-ranging connections, the DDCS has recently been implicated in a wide range of behaviors related to reward processing, aversion and motivation. As such, an understanding of the structure and connections of the DDCS may help illuminate the pathophysiology of neuropsychiatric disorders such as depression, addiction and pain. This is the first review of all three components of the DDCS, the stria medullaris, the habenula and the fasciculus retroflexus, with particular focus on their anatomy, function and development.

Tacr3 in the lateral habenula differentially regulates orofacial allodynia and anxiety-like behaviors in a mouse model of trigeminal neuralgia

Trigeminal neuralgia (TN) is debilitating and is usually accompanied by mood disorders. The lateral habenula (LHb) is considered to be involved in the modulation of pain and mood disorders, and the present study aimed to determine if and how the LHb participates in the development of pain and anxiety in TN. To address this issue, a mouse model of partial transection of the infraorbital nerve (pT-ION) was established. pT-ION induced stable and long-lasting primary and secondary orofacial allodynia and anxiety-like behaviors that correlated with the increased excitability of LHb neurons. Adeno-associated virus (AAV)-mediated expression of hM4D(Gi) in glutamatergic neurons of the unilateral LHb followed by clozapine-N-oxide application relieved pT-ION-induced anxiety-like behaviors but not allodynia. Immunofluorescence validated the successful infection of AAV in the LHb, and microarray analysis showed changes in gene expression in the LHb of mice showing allodynia and anxiety-like behaviors after pT-ION. Among these differentially expressed genes was Tacr3, the downregulation of which was validated by RT-qPCR. Rescuing the downregulation of Tacr3 by AAV-mediated Tacr3 overexpression in the unilateral LHb significantly reversed pT-ION-induced anxiety-like behaviors but not allodynia. Whole-cell patch clamp recording showed that Tacr3 overexpression suppressed nerve injury-induced hyperexcitation of LHb neurons, and western blotting showed that the pT-ION-induced upregulation of p-CaMKII was reversed by AAV-mediated Tacr3 overexpression or chemicogenetic inhibition of glutamatergic neurons in the LHb. Moreover, not only anxiety-like behaviors, but also allodynia after pT-ION were significantly alleviated by chemicogenetic inhibition of bilateral LHb neurons or by bilateral Tacr3 overexpression in the LHb. In conclusion, Tacr3 in the LHb plays a protective role in treating trigeminal nerve injury-induced allodynia and anxiety-like behaviors by suppressing the hyperexcitability of LHb neurons. These findings provide a rationale for suppressing unilateral or bilateral LHb activity by targeting Tacr3 in treating the anxiety and pain associated with TN.

Case Report: Lateral Habenula Deep Brain Stimulation for Treatment-Resistant Depression

Treatment-resistant depression (TRD) is a chronic and severe psychiatric illness associated with limited therapeutic options. Deep brain stimulation (DBS) is a promising therapy for TRD patients. However, its safety and efficacy are still unclear. Here we reported the safety and efficacy of lateral habenula (LHb) DBS for a TRD patient who had failed medical, psychological, electroconvulsive, and ketamine therapy. The DBS system is compatible with 3T magnetic resonance imaging along with local field potential (LFP) streaming. Two DBS electrodes were implanted at the bilateral LHb without any complication. The patient showed acute stimulation effects and achieved long-term improvements in his depression, anxiety, and sleep with left LHb 160 Hz frequency stimulation, accompanying the change of LFPs. These results provided clinical evidence toward the safety and efficacy and electrophysiological basis of LHb DBS for TRD.

Three-Dimensional Analysis of Mouse Habenula Subnuclei Reveals Reduced Volume and Gene Expression in the Lipopolysaccharide-mediated Depression Model

The habenula (Hb) is small but important brain structure, anatomically and functionally links the forebrain with the midbrain to modulate various neuropsychiatric functions associated with drug addiction and emotion-associated dysfunctions. Several reports suggested that the dysfunction of Hb-related functions affected the Hb structurally and functionally. However, the technical limitation has awaited the solid conclusion of whether Hb change due to depression is likely to occur in certain subnuclei of the Hb. To probe this possibility, we developed 3-dimensional reconstruction methods for the high-resolution volumetric analysis of Hb and the mRNA levels at the given volume in normal or lipopolysaccharide (LPS)-mediated mouse model of depression. Notably, we discovered that the volume reduction was prominent in medial Hb but not in lateral Hb after LPS treatments. On the other hand, the RNA expression levels of known Hb regional markers such as Tac1 (dorsal part of medial Hb), ChAT (ventral part of medial Hb), and Tacr1 (medial and lateral Hb) were all decreased in all Hb subnuclei in LPS-injected mice. Accordingly, accurate volumetry with marker labeling was not feasible. Collectively, these established 3D analyses of mouse Hb successfully and precisely determine the volume-based changes of small brain structure, which should be applicable in a wider range of mouse models or pathological specimens.

Habenular connections with the dopaminergic and serotonergic system and their role in stress-related psychiatric disorders

The habenula (Hb) is a phylogenetically old epithalamic structure differentiated into two nuclear complexes, the medial (MHb) and lateral habenula (LHb). After decades of search for a great unifying function, interest in the Hb resurged when it was demonstrated that LHb plays a major role in the encoding of aversive stimuli ranging from noxious stimuli to the loss of predicted rewards. Consistent with a role as an anti-reward center, aberrant LHb activity has now been identified as a key factor in the pathogenesis of major depressive disorder. Moreover, both MHb and LHb emerged as new players in the reward circuitry by primarily mediating the aversive properties of distinct drugs of abuse. Anatomically, the Hb serves as a bridge that links basal forebrain structures with monoaminergic nuclei in the mid- and hindbrain. So far, research on Hb has focused on the role of the LHb in regulating midbrain dopamine release. However, LHb/MHb are also interconnected with the dorsal (DR) and median (MnR) raphe nucleus. Hence, it is conceivable that some of the habenular functions are at least partly mediated by the complex network that links MHb/LHb with pontomesencephalic monoaminergic nuclei. Here, we summarize research about the topography and transmitter phenotype of the reciprocal connections between the LHb and ventral tegmental area-nigra complex, as well as those between the LHb and DR/MnR. Indirect MHb outputs via interpeduncular nucleus to state-setting neuromodulatory networks will also be commented. Finally, we discuss the role of specific LHb-VTA and LHb/MHb-raphe circuits in anxiety and depression.

Toward an understanding of the habenula's various roles in human depression

The habenula is an evolutionarily conserved structure in the vertebrate brain. Lesion and electrophysiological studies in animals have suggested that it is involved in the regulation of monoaminergic activity through projection to the brain stem nuclei. Since studies in animal models of depression and human functional imaging have indicated that increased activity of the habenula is associated with depressive phenotypes, this structure has attracted a surge of interest in neuroscience research. According to pathway- and cell-type-specific dissection of habenular function in animals, we have begun to understand how the heterogeneity of the habenula accounts for alteration of diverse physiological functions in depression. Indeed, recent studies have revealed that the subnuclei embedded in the habenula show a wide variety of molecular profiles not only in neurons but also in glial cells implementing the multifaceted regulatory mechanism for output from the habenula. In this review, we overview the known facts on mediolateral subdivision in the habenular structure, then discuss heterogeneity of the habenular structure from the anatomical and functional viewpoint to understand its emerging role in diverse neural functions relevant to depressive phenotypes. Despite the prevalent use of antidepressants acting on monoamine metabolisms, ~30% of patients with major depression are reported to be treatment-resistant. Thus, cellular mechanisms deciphering such diversity in depressive symptoms would be a promising candidate for the development of new antidepressants.

Detailed mapping of human habenula resting-state functional connectivity

The habenula (Hb) inhibits dopaminergic reward signaling in response to negative outcomes and has been linked to numerous functional domains relevant to mental health, including reward prediction, motivation, and aversion processing. Despite its important neuroscientific and clinical implications, however, the human Hb remains poorly understood due to its small size and the associated technical hurdles to in vivo functional magnetic resonance imaging (fMRI) investigation. Using high-resolution 3 T fMRI data from 68 healthy young adults acquired through the Human Connectome Project, we developed a rigorous approach for mapping the whole-brain resting-state functional connectivity of the human Hb. Our study combined an optimized strategy for defining subject-level connectivity seeds to maximize Hb blood-oxygen-level-dependent (BOLD) signal sensitivity with high-quality surface-based alignment for robust functional localization and cortical sensitivity. We identified significant positive Hb connectivity with: (i) conserved brainstem targets, including the dopaminergic ventral tegmental area, serotonergic raphe nuclei, and periaqueductal gray; (ii) subcortical structures related to reward and motor function, including the nucleus accumbens, dorsal striatum, pallidum, thalamus, and cerebellum; and (iii) cortical areas associated with the Salience Network and early sensory processing, including the dorsal anterior cingulate, anterior insula, and primary visual and auditory cortices. Hb connectivity was strongly biased towards task-positive brain regions, with weak or negative connectivity observed throughout the task-negative Default Mode Network. Our study provides a detailed characterization of Hb resting-state functional connectivity in healthy young adults, demonstrating both the feasibility and clinical potential of studying the human Hb using high-resolution 3 T fMRI.

Awakening Neuropsychiatric Research Into the Stria Medullaris: Development of a Diffusion-Weighted Imaging Tractography Protocol of This Key Limbic Structure

The Stria medullaris (SM) Thalami is a discrete white matter tract that directly connects frontolimbic areas to the habenula, allowing the forebrain to influence midbrain monoaminergic output. Habenular dysfunction has been shown in various neuropsychiatric conditions. However, there exists a paucity of research into the habenula’s principal afferent tract, the SM. Diffusion-weighted tractography may provide insights into the properties of the SM in vivo, opening up investigation of this tract in conditions of monoamine dysregulation such as depression, schizophrenia, addiction and pain. We present a reliable method for reconstructing the SM using diffusion-weighted imaging, and examine the effects of age and gender on tract diffusion metrics. We also investigate reproducibility of the method through inter-rater comparisons. In consultation with neuroanatomists, a Boolean logic gate protocol was developed for use in ExploreDTI to extract the SM from constrained spherical deconvolution based whole brain tractography. Particular emphasis was placed on the reproducibility of the tract, attention to crossing white matter tract proximity and anatomical consistency of anterior and posterior boundaries. The anterior commissure, pineal gland and mid point of the thalamus were defined as anatomical fixed points used for reconstruction. Fifty subjects were scanned using High Angular Resolution Diffusion Imaging (HARDI; 61 directions, b-value 1500 mm³). Following constrained spherical deconvolution whole brain tractography, two independent raters isolated the SM. Each output was checked, examined and cleaned for extraneous streamlines inconsistent with known anatomy of the tract by the rater and a neuroanatomist. A second neuroanatomist assessed tracts for face validity. The SM was reconstructed with excellent inter-rater reliability for dimensions and diffusion metrics. Gender had no effect on the dimensions or diffusion metrics, however radial diffusivity (RD) showed a positive correlation with age. Reliable identification and quantification of diffusion metrics of the SM invites further exploration of this key habenula linked structure in neuropsychiatric disorders such as depression, anxiety, chronic pain and addiction. The accurate anatomical localization of the SM may also aid preoperative stereotactic localization of the tract for deep brain stimulation (DBS) treatment.

Reproducibility of myelin content-based human habenula segmentation at 3 Tesla

In vivo morphological study of the human habenula, a pair of small epithalamic nuclei adjacent to the dorsomedial thalamus, has recently gained significant interest for its role in reward and aversion processing. However, segmenting the habenula from in vivo magnetic resonance imaging (MRI) is challenging due to the habenula's small size and low anatomical contrast. Although manual and semi-automated habenula segmentation methods have been reported, the test-retest reproducibility of the segmented habenula volume and the consistency of the boundaries of habenula segmentation have not been investigated. In this study, we evaluated the intra- and inter-site reproducibility of in vivo human habenula segmentation from 3T MRI (0.7-0.8 mm isotropic resolution) using our previously proposed semi-automated myelin contrast-based method and its fully-automated version, as well as a previously published manual geometry-based method. The habenula segmentation using our semi-automated method showed consistent boundary definition (high Dice coefficient, low mean distance, and moderate Hausdorff distance) and reproducible volume measurement (low coefficient of variation). Furthermore, the habenula boundary in our semi-automated segmentation from 3T MRI agreed well with that in the manual segmentation from 7T MRI (0.5 mm isotropic resolution) of the same subjects. Overall, our proposed semi-automated habenula segmentation showed reliable and reproducible habenula localization, while its fully-automated version offers an efficient way for large sample analysis.

Neurogenetic asymmetries in the catshark developing habenulae: mechanistic and evolutionary implications

Analysis of the establishment of epithalamic asymmetry in two non-conventional model organisms, a cartilaginous fish and a lamprey, has suggested that an essential role of Nodal signalling, likely to be ancestral in vertebrates, may have been largely lost in zebrafish. In order to decipher the cellular mechanisms underlying this divergence, we have characterised neurogenetic asymmetries during habenular development in the catshark Scyliorhinus canicula and addressed the mechanism involved in this process. As in zebrafish, neuronal differentiation starts earlier on the left side in the catshark habenulae, suggesting the conservation of a temporal regulation of neurogenesis. At later stages, marked, Alk4/5/7 dependent, size asymmetries having no clear counterparts in zebrafish also develop in neural progenitor territories, with a larger size of the proliferative, pseudostratified neuroepithelium, in the right habenula relative to the left one, but a higher cell number on the left of a more lateral, later formed population of neural progenitors. These data show that mechanisms resulting in an asymmetric, preferential maintenance of neural progenitors act both in the left and the right habenulae, on different cell populations. Such mechanisms may provide a substrate for quantitative variations accounting for the variability in size and laterality of habenular asymmetries across vertebrates.

Bsx controls pineal complex development

Neuroendocrine cells in the pineal gland release melatonin during the night and in teleosts are directly photoreceptive. During development of the pineal complex, a small number of cells migrate leftward away from the pineal anlage to form the parapineal cell cluster, a process which is crucial for asymmetrical development of the bilateral habenular nuclei. Here we show that, throughout zebrafish embryonic development, the brain-specific homeobox (bsx) gene is expressed in all cell types of the pineal complex. We identified Bmp and Noto/Flh as major regulators of bsx expression in the pineal complex. Upon loss of Bsx through the generation of a targeted mutation, embryos fail to form a parapineal organ and develop right-isomerized habenulae. Crucial enzymes in the melatonin biosynthesis pathway are not expressed, suggesting absence of melatonin from the pineal gland of bsx mutants. Several genes involved in rod-like or cone-like phototransduction are also abnormally expressed, indicating that Bsx plays a pivotal role in differentiation of multiple cell types in the zebrafish pineal complex.

Imaging Habenula Volume in Schizophrenia and Bipolar Disorder

The habenula (Hb), a bilateral nucleus located next to the dorsomedial thalamus, is of particular relevance to psychiatric disorders based on preclinical evidence linking the Hb to depressive and amotivational states. However, studies in clinical samples are scant because segmentation of the Hb in neuroimaging data is challenging due to its small size and low contrast from the surrounding tissues. Negative affective states dominate the clinical course of schizophrenia and bipolar disorder and represent a major cause of disability. Diagnosis-related alterations in the volume of Hb in these disorders have therefore been hypothesized but remain largely untested. To probe this question, we used a recently developed objective and reliable semi-automated Hb segmentation method based on myelin-sensitive magnetic resonance imaging (MRI) data. We ascertained case-control differences in Hb volume from high resolution structural MRI data obtained from patients with schizophrenia (n = 95), bipolar disorder (n = 44) and demographically matched healthy individuals (n = 52). Following strict quality control of the MRI data, the final sample comprised 68 patients with schizophrenia, 32 with bipolar disorder and 40 healthy individuals. Regardless of diagnosis, age, sex, and IQ were not correlated with Hb volume. This was also the case for age of illness onset and medication (i.e., antipsychotic dose and lithium-treatment status). Case-control differences in Hb volume did not reach statistical significance; their effect size (Cohen's d) was negligible on the left (schizophrenia: 0.14; bipolar disorder: -0.03) and small on the right (schizophrenia: 0.34; bipolar disorder: 0.26). Nevertheless, variability in the volume of the right Hb was associated with suicidality in the entire patient sample (ρ = 0.29, p = 0.004) as well as in each patient group (bipolar disorder: ρ = 0.34, p = 0.04; schizophrenia: ρ = 0.25, p = 0.04). These findings warrant replication in larger samples and longitudinal designs and encourage more comprehensive characterization of Hb connectivity and function in clinical populations.

The art of matching brain tissue from patients and controls for postmortem research

The quality of postmortem research depends strongly on a thorough clinical investigation and documentation of the patient's disorder and therapies. In addition, a systematic and professional neuropathologic investigation of both cases and controls is absolutely crucial. In the experience of the Netherlands Brain Bank (NBB), about 20% of clinical neurologic diagnoses, despite being made in first-rate clinics, have to be revised or require an extra diagnosis after a complete and thorough review by the NBB. The neuropathology examination may reveal for instance that the "controls" already have preclinical neurodegenerative alterations. In postmortem studies the patient and control groups must be matched for as many of the known confounding factors as possible. This is necessary to make the groups as similar as possible, except for the topic being investigated. Confounding factors are present before, during, and after death. They are respectively: (1) genetic background, systemic diseases, duration and gravity of illness, medicines and addictive compounds used, age, sex, gender identity, sexual orientation, circadian and seasonal fluctuations, lateralization; (2) agonal state, stress of dying; and (3) postmortem delay, freezing procedures, fixation and storage time. Consequently, a brain bank should have a large number of controls at its disposal for appropriate matching. If matching fails for some confounders, then their influence may be determined by statistical methods such as analysis of variance or regression models.

Nodal signalling and asymmetry of the nervous system

The role of Nodal signalling in nervous system asymmetry is still poorly understood. Here, we review and discuss how asymmetric Nodal signalling controls the ontogeny of nervous system asymmetry using a comparative developmental perspective. A detailed analysis of asymmetry in ascidians and fishes reveals a critical context-dependency of Nodal function and emphasizes that bilaterally paired and midline-unpaired structures/organs behave as different entities. We propose a conceptual framework to dissect the developmental function of Nodal as asymmetry inducer and laterality modulator in the nervous system, which can be used to study other types of body and visceral organ asymmetries. Using insights from developmental biology, we also present novel evolutionary hypotheses on how Nodal led the evolution of directional asymmetry in the brain, with a particular focus on the epithalamus. We intend this paper to provide a synthesis on how Nodal signalling controls left-right asymmetry of the nervous system.This article is part of the themed issue 'Provocative questions in left-right asymmetry'.

Development and connectivity of the habenular nuclei

Accumulating evidence has reinforced that the habenular region of the vertebrate dorsal forebrain is an essential integrating center, and a region strongly implicated in neurological disorders and addiction. Despite the important and diverse neuromodulatory roles the habenular nuclei play, their development has been understudied. The emphasis of this review is on the dorsal habenular nuclei of zebrafish, homologous to the medial nuclei of mammals, as recent work has revealed new information about the signaling pathways that regulate their formation. Additionally, the zebrafish dorsal habenulae have become a valuable model for probing how left-right differences are established in a vertebrate brain. Sonic hedgehog, fibroblast growth factors and Wingless-INT proteins are all involved in the generation of progenitor cells and ultimately, along with Notch signaling, influence habenular neurogenesis and left-right asymmetry. Intriguingly, a genetic network has emerged that leads to the differentiation of dorsal habenular neurons and, through localized chemokine signaling, directs the posterior outgrowth of their newly emerging axons towards their postsynaptic target, the midbrain interpeduncular nucleus.

The medial habenula and interpeduncular nucleus circuitry is critical in addiction, anxiety, and mood regulation

Abstinence from chronic use of addictive drugs triggers an aversive withdrawal syndrome that compels relapse and deters abstinence. Many features of this syndrome are common across multiple drugs, involving both affective and physical symptoms. Some of the network signaling underlying withdrawal symptoms overlaps with activity that is associated with aversive mood states, including anxiety and depression. Given these shared features, it is not surprising that a particular circuit, the dorsal diencephalic conduction system, and the medial habenula (MHb) and interpeduncular nucleus (IPN), in particular, have been identified as critical to the emergence of aversive states that arise both as a result and, independently, of drug addiction. As the features of this circuit continue to be characterized, the MHb-IPN axis is emerging as a viable target for therapeutics to aid in the treatment of addiction to multiple drugs of abuse as well as mood-associated disorders. This is an article for the special issue XVth International Symposium on Cholinergic Mechanisms.